Abstract

A preliminary landslide location map was prepared as part of a U.S. Geological Survey investigation of landslide hazards in Puerto Rico in cooperation with the Puerto Rico Planning Board. Presented here are preliminary observations based on examination of aerial photography (years 1936, 1951, 1964, 1972, 1979, 1986, 1988) and field surveys performed in 1988 and 1989. On the basis of the time intervals between sequential photographic sets, seven age classes were developed; most of the mapped landslides are active, and the ages given are for the earliest estimated date of failure. Of the 173 landslides mapped, all but 20 were field verified; there are 107 landslide that are associated with highway construction and 66 natural landslides that occurred on forested slopes. Landslide aspect and angle were measured with a Brunton compass, and areas were calculated from measurements by tape, pace, or rangefinder.

The Caribbean National Forest covers 77 percent of the area shown in the El Yunque quadrangle and contains all the mapped landslides. Mean annual temperature and rainfall range from 26.5 ° C and 2,450 mm in the lower elevations to 17.5 ° C and more than 4,000 mm at higher elevations (Brown and others, 1983). Maximum rainfall occurs from May to November, although intense rainfall may occur throughout the year and is commonly associated with tropical depressions or hurricanes. A 2-day storm with a 2-year recurrence interval produces as much as 230 mm of rain, and a 2-day storm with a 5-year recurrence interval can produce as much as 280 mm (Miller, 1965). Landslides are a common phenomena in the Caribbean National Forest and normally occur during periods of high rainfall. Bedrock in the area is dominated by Cretaceous marine-deposited andestic to basaltic volcaniclastics (Seiders, 1971). The Rio Blanco Stock, a light-gray medium- to coarse-grained quartz diorite, was intruded into the area shown in the south-east section of the quadrangle apparently in the Paleocene Epoch (see map for boundary). The intrusion covers about 20 km2 and is bounded by a zone of metamorphism that extends 3.5 km into the surrounding volcaniclastics. Slopes in the quadrangle are mantled with silty-clay saprolitic soils that, based on observations at road cuts, slide-scarp exposures, drill logs, seismic refraction surveys (Larsen, 1989) and previous work (Dames and Moore, 1980), range in thickness from 2 to 20 m. The number of mapped landslides represents a conservative estimate of the total number likely too exist in the area because the photographs are of uneven quality. Also, rapid revegetation tends to mask landslides older than 20 years on aerial photographs. Complete recovery of landslide areas may take a minimum of 60 years for natural landslides (Guariguata, 1990), and even longer for landslides associated with highway construction because they to tend to remain active for greater periods.

After field work was completed for this map, Hurricane Hugo struck eastern Puerto Rico in September, 1989, with maximum sustained winds of over 225 km per hour. The 3-day rainfall total associated with the hurricane ranged from 165 to 344 mm. Over 200 landslides occurred within and near the El Yunque quadrangle; most of these were small, shallow earth slides and debris flows (classification after Varnes, 1978)(Larsen, 1990).